I wrote a little ruby (nice interpretive language) wrapper round the C
port of the nec2 program, and began happily generating simulations. So
far so good. (When I am finished I could offer it to anyone interested)

I am new to EM stuff - so here comes what is probably a silly
misunderstanding:

Consider an array of similar aerials (antennae).

Since the situation is linear, I would expect the currents received from a
plane wave signal to be linearly related to the currents which would be
obtained at each element position if there were no aerials at the other
element positions (assuming the effect of an aerial on its surroundings is
due solely to its current, and you can linearly add a cancelling current
and thus remove it)?

The relationship can be expressed by a symmetric (reciprocity) coupling
matrix which should be independant of the incident plane wave direction?

For a regular array on a circle the matrix should be toeplitz, by
symmetry, and constitutes a cyclic FIR filter which will yield the coupled
signal observations from the (free) field values at the element positions
(to a complex scale factor). It can thus be found by division of the FFTs
(of the loaded simulation currents vs open circuit simulation currents
or vs free field plane wave calculation) and inverse FFT.

When I try this - for example with a regular 3 element circular array of
resonant dipoles - I find that the FIR coefficients derived do indeed
convert from free field pattern at the element positions to array
currents, but only for the angle from which they are derived. The
relationship seems to be signal dependant :-(. For example the plane waves
in the opposite sense (180 deg) have a quite different coupling matrix
(mostly in phase).

Can anyone point out my mistake, this is really bugging me. I have tried
increasing the number of segments to silly levels, to no avail. For 2
dipoles it works, more and it does not.

Help